The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system of the vehicle from two-wheel drive to four-wheel drive when one of the wheels of the vehicle slips.
In a conventional four-wheel drive vehicle, a power transmission system for the two-wheel driving is selectively converted to the four-wheel driving system by engaging a clutch which is manually operated by a select lever.
When the vehicle travels on slippery, icy or snowy roads by two-wheel driving, the transmission system should be changed to the four-wheel drive in order to prevent wheels from slipping. If the changing operation is done after the slipping because of driver's misjudgement on the slipping, the slipping cannot be stopped or reduced. Therefore, it is necessary to change the transmission system to the four-wheel drive at the initial stage of the slipping.
To meet such a requirement, an automatic control system for the four-wheel drive system has been proposed in Japanese patent application No. 56-106648 which was filed by the assignee of this application. The system is provided with a slip detecting circuit which produces a clutch signal when the difference between speeds of front and rear wheels exceeds a predetermined reference value. The clutch signal causes a clutch to engage, so that the two-wheel drive is automatically changed to the four-wheel drive.
In this connection, it should be noted that the speed difference between front and rear wheels at the beginning of the slipping increases as the load on the engine increases. Therefore, if the reference value for detecting the slipping is set to a high value, slipping at light load driving cannot be detected. FIG. 3 shows the relation between vehicle speed, engine power and throttle valve position. From the graph, it will be seen that the throttle valve of the engine is fully opened at heavy load, for example at steep uphill driving, and the throttle valve is slightly opened at light load such as downhill driving in spite of the same vehicle speed. Thus, it will be seen that the above mentioned reference value should be changed according to the variation of the load.